Drosophila motor neurons continuously sprout new synaptic boutons during muscle growth and development. Bouton expansion is regulated by local changes in FasII, a homotypic Ig family adhesion molecule, and co-first authors James Ashley and Mary Packard showed they could double the number of bouton buds in a given axon by overexpressing FasII. Since the budding was reminiscent of what they had previously seen in APPL-overexpressing flies, they wondered whether APPL might be responsible for FasII signaling.

By using fly strains that over- or underexpressed a variety of FasII or APPL alleles, Ashley and Packard determined that APPL was in fact required for FasII to increase the formation of boutons at the NMJ, and that APPL functioned downstream of the adhesion molecule in a signaling pathway. Increased bouton formation also required the PDZ domain-containing protein dX11, the fly homolog of a mammalian cytosolic adaptor protein that associates with the APP cytoplasmic domain. Immunoprecipitation experiments showed that FasII, APPL, and dX11 were in direct association with each other in muscle cell extracts.

From their own analysis and work of others, Ashley et al. propose a model where homophilic binding of FasII located on the pre- and postsynaptic cells brings dX11 and APPL to presynaptic sites of cell adhesion. dX11 carries with it exocytic machinery which could add new membrane to the bud, while APPL might function via G proteins to stimulate microtubule extension.

In support of this model, the researchers show that FasII overexpression only increases synapse formation when the protein is present on both sides of the neuromuscular junction—imbalanced elevation of FasII in neurons only interfered with synapse formation, and prevented APPL-induced synapse development. Interestingly, imbalanced expression of FasII, especially in concert with a gain-of-function APPL allele, caused formation of large, morphologically abnormal boutons. These giant boutons contained excess internal membrane structures, abnormal microtubule clusters, and aberrantly accumulated APPL and FasII. “These internal APPL accumulations within the boutons seem reminiscent of intraneuronal amyloid-β accumulation,” the authors write, and may “provide additional clues toward a mechanism by which interference with normal APP function could lead to pathological events and subsequent symptoms of Alzheimer’s disease.”—Pat McCaffrey.

Reference:
Ashley J, Packard M, Ataman B, Budnik V. Fasciclin II signals new synapse formation through amyloid precursor protein and the scaffolding protein dX11/Mint. Journal of Neuroscience. 2005 June 22; 25:5943-5955.